A major challenge of modern cancer immunotherapy is the identification of cytotoxic T lymphocyte (CTL) epitopes from defined tumor-associated antigens (TAA) that promote lysis of tumor cells. The majority of antigens on human cancers are not tumor specific and are overexpressed in malignant cells as opposed to cells of normal tissues. Therefore, immunity to cancer in humans may rest mostly on the development of an effective immune response mainly directed to self-molecules qualitatively common to all cell types.
Human carcinoembryonic antigen (CEA) is a 180 kD glycoprotein expressed on the majority of colon, rectal, stomach and pancreatic tumors (1), some 50% of breast carcinomas (2) and 70% of lung carcinomas (3). CEA is also expressed in fetal gut tissue, and to a lesser extent on normal colon epithelium. The immunogenicity of CEA has been ambiguous, with several studies reporting the presence of anti-CEA antibodies in patients (4-7) while other studies have not (8-10). CEA was first described as a cancer specific fetal antigen in adenocarcinoma of the human digestive tract in 1965 (Gold, P. and Freeman, S. O. (1965) Exp. Med. 121:439-462). Since that time, CEA has been characterized as a cell surface antigen produced in excess in nearly all solid tumors of the human gastrointestinal tract. The gene for the human CEA protein has been cloned. (Oikawa et al (1987) Biochim. Biophys. Res. 142:511-518; European Application No. EP 0346710).
Recently, the first evidence was reported of a human CTL response to CEA (11). This CAP1 peptide showed the highest level of T2 cell binding among the various CEA peptides tested with stimulation of the T cells resulting in the generation of cytotoxic T cell lines. We have identified a 9-mer peptide, designated CAP1 (with the sequence YLSGANLNL) (SEQ. ID NO: 1), on the basis of binding to HLA-A2, and the ability to generate specific CTL from peripheral blood mononuclear cells (PBMC) from carcinoma patients immunized with a recombinant vaccinia virus expressing CEA (rV-CEA). For example, peripheral blood lymphocytes (PBLs) from 5 patients showed signs of T cell response to CAP1 peptide after immunization with rV-CEA. Two other laboratories have since generated CAP1 specific CTL in vitro employing peptide pulsed dendritic cells as antigen presenting cells (APC) (12). It has also recently been reported (13) that CAP1 specific CTL can be generated from PBMC from carcinoma patients immunized with the avipox recombinant ALVAC-CEA. Several groups have also reported the generation of anti-CEA antibodies and CEA specific proliferative T cell responses following immunization with either an anti-Id to an anti-CEA monoclonal antibody (MAb) (14), recombinant CEA protein (15), or rV-CEA (16).
Several investigators have introduced CTL to tumor associated and viral antigens by in vitro stimulation of PBMC with an immunodominant peptide. Recent work with the gp100 melanoma antigen (17-19), an HIV polymerase peptide (20) and the papilloma virus tumor antigen E6 (21) demonstrated enhanced immunogenicity after modifications to the peptide sequences. In these studies, replacements were at anchor positions and were intended to increase binding to murine or human MHC antigens. This approach was based on a demonstrated correlation between immunogenicity and peptide binding affinity to class I MHC (major histocompatibility complex) molecules for viral antigen epitopes (22).
Previous investigators have also worked with fragments of CEA. Thus, Shively (1989), in a European patent publication (EP No. 0343946 A2) reports a number of CEA fragments that include a unique epitope (as defined by its reactivity with an antibody). The latter CEA fragment is 177 amino acid residues long and contains the 9-mer sequence of CAP1. However, no shorter CEA fragments that include the CAP1 sequence were described.
In sum, the use of rV-CEA alone as an agent for boosting the CEA-specific immune response of rV-CEA suffers from the drawback of stimulating an immune response to vaccinia virus. However, the novel combination of rV-CEA and CAP1 suggested itself to us as a “second generation protocol” for the treatment of cancer patients.
It is an accepted principle that when an immunogenic peptide is modified in a conserved manner (e.g., a hydrophobic amino acid is substituted with a hydrophobic amino acid) the modified peptide is likely to have similar immunogenic activity based upon the maintenance of the molecule's shape, charge and hydrophobic character.
More specifically, a study by Madden (33) has identified specific amino acid preferences in peptides for MHC-complexing, a precursor step to T cell recognition. Madden as well as other investigators (31) suggest that specific amino acid positions in peptides are available for T cell recognition.
Skipper et al. (40) describes the identification and characterization of a naturally-occurring peptide epitope of tyrosinase, wherein the peptide sequence differs from that which is predicted from the DNA. This modified peptide is recognized by tyrosinase-specific human cytotoxic T-lymphocytes (“CTL”) more effectively than the direct translation product and is the only one of the two peptides to be presented by HLA-A2.1 molecules on the cell surface. The modification is a substitution of an asparagine with an aspartic acid. The authors propose that the asparagine is N-glycosylated in the endoplasmic reticulum during protein synthesis and is deamidated post-translationally.
In the case of CAP1, the primary and secondary anchors at positions 2, 9, and 1 are already occupied by preferred amino acids and so a different approach was taken to improve peptide immunogenicity by attempting to enhance its ability to bind to the TCR. It appeared to us that by altering amino acid residues expected to contact the TCR one could generate an analog of CAP1 with substitutions at non-MHC anchor positions. Such an analog might then represent a T cell enhancer agonist capable of stimulating CTL more efficiently than the native peptide. Previous results supported the concept that some peptide analogs could act as T cell antagonists by inhibiting responses to the antigenic peptide (23-29). Such inhibition was shown to be TCR specific and could not be explained by competition for peptide binding to the MHC protein. Analogously, a peptide enhancer agonist would be an analog that increased the effector function without accompanying increases in MHC binding. We therefore sought to increase CAP1 immunogenicity by analyzing panels of analogs containing single amino acid substitutions to residues we predicted would interact with the T cell receptor (TCR) of CAP1-specific CTL. The present invention relates to the construction of a novel T cell enhancer agonist for the CAP1 peptide, the first such example for a human CTL epitope.